The present invention relates to a constant current circuit using a current mirror circuit, preferably employed in a throttle control comparator for an electronic control unit of an internal combustion engine or an anti-skid control apparatus of an automotive vehicle.
Unexamined Japanese Patent Publication No. 1-21616, corresponding to the U.S. Pat. No. 4,897,614, discloses a conventional constant current circuit using a current-mirror circuit. FIG. 8 shows a fundamental circuit arrangement of this conventional constant current circuit.
This conventional constant current circuit comprises a constant current source 101 supplying a constant current I1. Two NPN transistors 102 and 103 cooperatively constitute a current mirror circuit 104 receiving the current I1 supplied from the constant current source 101. The current I2 flowing from the power source 105 is made constant by the current mirror circuit 104. At a predetermined portion in the circuit, a reference voltage Vref is detectable as a value smaller than a power source voltage VB by a constant amount equivalent to a voltage drop at a resistor 106.
According to this conventional constant current circuit, the current I2 flows across the transistor 103 when the constant current source 101 supplies the constant current I1 to the transistor 102. However, the current I2 is responsive to a variation of the power source voltage VB.
FIG. 9 shows a relationship between the power source voltage VB and the currents I1 and I2. As apparent from the characteristics shown in FIG. 9, the current I2 increases in accordance with an increase of the power source voltage VB due to a so-called early effect of the transistor 103. The early effect is a phenomenon that a depletion layer of a transistor is widened depending on a bias voltage between collector and base terminals. When the collector-emitter voltage increases, the collector current increases even when the base current is constant. For example, when the power source voltage VB changes from 6V to 20V, the current I2 increases from 650 .mu.A to 850 .mu.A.
The voltage drop at the resistor 106 varies in proportion to the variation of the current I2. This gives an adverse influence to the detectable reference voltage Vref.